1. Field of the Invention:
This invention relates to document feeders and, more specifically, to document feeders useful for feeding seriatim document sheets to a platen at an exposure station of a copier/duplicator. More particularly, the invention relates to such a document feeder having an improved vacuum system for a sheet transport.
2. Description of the Prior Art:
Various types of document feeders or copier/duplicators are well known in the art. For example, commonly assigned U.S. Pat. No. 4,169,674, entitled Recirculating Sheet feeder, which issued on Oct. 2, 1979 in the name of Matthew J. Russel discloses a recirculating sheet feeder wherein a stack of document sheets to be fed to a platen of a copier/duplicator is placed in a tray. An oscillating vacuum feeder removes the sheets seriatim from the bottom of the stack for transport of the sheet by various rollers to a registration position on the platen for copying. After exposure the sheet is returned to the stack on top of the other sheets remaining in the stack.
Commonly assigned U.S. Pat. No. 4,176,945, entitled Sheet Feeding Apparatus for Use With Copiers/Duplicators or the Like, which issued on Dec. 4, 1979 in the names of R. C. Holzhauser et al discloses recirculating sheet feeder wherein provision is made for inverting a document sheet and returning it to the platen for copying of a second side of the document sheet prior to returning the sheet to the top of the stack. In this manner both sides of a document sheet can be copied. The Holzhauser et al patent also dicloses document positioner apparatus whereby an individual sheet is fed to the platen, copied one or more times and removed from the platen without being fed along the entire recirculating sheet path leading from the tray to the platen and back to the tray.
It is also known to provided recirculating document feeders with vacuum sheet transports for movement of a document sheet across the platen to a registration position. In this regard, see commonly assigned U.S. Pat. No. 4,179,215, entitled RECIRCULATING DOCUMENT FEEDER, which issued on Dec. 18, 1979 in the name of C. T. Hage. A combination document feeder and positioner with a platen vacuum transport is disclosed in Item 18540 at pages 526 and 527 of the September 1979 edition of Research Disclosure, a publication of Industrial Opportunities, Ltd., Homewell, Havant, Hamshire, PO91EF, United Kingdom.
A document loading and registration apparatus is disclosed at pages 213 and 214 of the March/April, 1979 edition of the Xerox disclosure Journal. The apparatus has a vacuum belt that travels over a vacuum chamber. The chamber can be separated into two sections by a movable damper or baffle that is located at a registration point. Initially, the damper is closed to isolate one section of the chamber from a vacuum blower. A document sheet is delivered to the portion of the vacuum belt above the isolated section and registered by fingers above the belt and damper. Then the damper is moved so that both sections of the chamber communicate with the vacuum blowers, and the belt is advanced across the vacuum chamber to move the sheets to a loading station.
In some of the prior art devices described above drive rollers are used for advancing sheets across the platen and against a registration member. The drive rollers continue to be driven after the sheet reaches the registration point and thereby slip on the sheet. This allows the sheet to adjust itself into a registered position and thereby eliminate skew that may have developed in the sheet as it was moved from the stack of sheets to the registration member. Generally, this continued driving of the sheet against the registration members does not adversely affect the sheet. However, in vacuum platen transports as disclosed, for example, in the beforementioned Research Disclosure Publication, the sheet may be gripped against the vacuum belt with a relatively high vacuum force. If the belt continue to drive the sheet after the sheet reaches the registration member, there may be some damage to the leading edge of the sheet, depending upon the nature of the sheet and driving force applied to the sheet. Even so, vacuum transports are desireable because they tend to minimize or eliminate skewing of the sheet as it is transported across the platen toward the registration position. Damage to the sheet can be minimized by reducing the level of vacuum applied to the vacuum belt so that the belt can move relative to the sheet after the sheet as been stopped by the registration member. However, when this occurs the vacuum transport may encounter difficulty in initially lifting the sheet off the platen and onto the belt of the vacuum transport. Thus there is a need for a platen vacuum transport which is capable of insuring that the document sheet is lifted to the transport and, at the same time, can drive the document sheet against a registration member without damaging the sheet.